The invention relates to diene polymers or copolymers having improved raw polymer viscosity which are adapted to form elastomer compositions having reduced hysteresis properties and tire treads having reduced rolling resistance. More particularly, the invention relates to diene polymers or copolymers which comprise a mixture of diene polymer or copolymer chains containing carbon-tin bonds in the main polymer or copolymer chains and diene polymer or copolymer chains containing terminals derived from fused-ring polynuclear aromatic compounds.
In recent years, those active in the tire industry have greatly increased their emphasis on the development of tires having both reduced rolling resistance and good wet traction properties. As is well known, that portion of the tire which exerts the greatest influence on rolling resistance and traction is the tread or tread rubber portion. Low rolling resistance is desirable from a fuel consumption standpoint while good wet traction is desirable from a safety standpoint. However, as a general rule, these properties have been found to conflict with each other. Thus, a reduction in rolling resistance generally leads to an almost directionally proportional reduction in wet traction while an increase in wet traction generally leads to an almost directionally proportional increase in rolling resistance.
The prior art has proposed a number of approaches to the solution of this problem. Such approaches have generally involved modifying the properties of the elastomer or elastomer composition utilized to form the tire tread in order to achieve the best possible balance between rolling resistance and traction. The approaches involving modification of the elastomer have generally been based on improving the interaction between the elastomer and the carbon black used in compounding the elastomer to prepare the tire tread composition in order to improve the dispersion of the carbon black into the elastomer. This has the effect of reducing the hysteresis of the elastomer composition which in turn results in lower rolling resistance of the treads formed therefrom.
One known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of elastomer compositions formed therefrom involves coupling the living diene polymer or copolymer chains with metal halides. Thus, U.S. Pat. Nos. 4,383,085 and 4,515,922 describe the coupling of living diene polymer or copolymer chains obtained by anionic polymerization using an organolithium initiator with metal halides such as tin halides, silicon halides and the like. These patents indicate that tire treads formed from rubber compositions containing the coupled polymers have reduced hysteresis along with reduced rolling resistance and improved wet skid resistance.
Another known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of the elastomer composition involves terminating the living diene polymer or copolymer chains with certain compounds containing functional groups which are reactive with the lithium terminals of the living polymer. Thus, living diene polymers or copolymers have been terminated for that purpose by reaction with compounds such as carbodiimides, halogenated nitriles, heterocyclic nitrogen compounds, alkyl benzoates, certain substituted imines and organic acid halides as well as compounds containing ##STR1## wherein X, Y, Z and the various R groups are organic groups such as aryl, substituted aryl, alkyl, substituted alkyl, cycloalkyl etc.
For example, U.S. Pat. No, 4,935,471 discloses a process for preparing a polydiene having a high level of affinity for carbon black which comprises reacting a metal terminated polydiene with a capping agent selected from the group consisting of (a) halogenated nitriles having the structural formula X--A--C.tbd.N wherein X represents a halogen atom and wherein A represents an alkylene group containing from 1 to 20 carbon atoms, (b) heterocyclic aromatic nitrogen containing compounds and (c) alkyl benzoates. The only halogenated nitrile compound specifically disclosed is chloroacetonitrile.
Diene polymer or copolymer elastomers containing a mixture of coupled polymer chains and certain terminally functionalized polymer chains and a method for their preparation are also known in the art. Thus, U.S. Pat. No. 4,616,069 discloses a process for making a diene polymer rubber which comprises reacting an active diene polymer rubber having alkali metal and/or alkaline earth metal terminals, with: (1) a tin compound expressed by the general formula R.sub.a SnX.sub.b (in which R stands for an alkyl, alkenyl, cycloalkyl or aromatic hydrocarbon group; X is a halogen atom, a is an integer of 0-2, and b is an integer of 2-4), and (2) at least one organic compound selected from the group consisting of aminoaldehydes, aminoketones, aminothioaldehydes, aminothioketones and the organic compounds having in their molecules ##STR2## linkages in which A stands for an oxygen or sulfur atom.
Organic compounds containing such linkages which are disclosed include various amide compounds, imide compounds, lactam compounds, urea compounds, carbamic acid derivatives and the corresponding sulfur-containing compounds.
The patent discloses that the order of the reaction with the tin compounds (1) and organic compounds (2) is optional, i.e. they may be performed sequentially by optional order or they may be performed simultaneously. The reference further discloses that the rubber material of the invention shows well-balanced rolling resistance (rebound) and wet skid resistance and also good processability and storage stability.
Diene polymer and copolymer elastomers described in the aforementioned patents possess certain advantages in important properties such as reduced hysteresis (i.e. lower rolling resistance) and good traction and, in certain instances, good processability. However, those skilled in the rubber and tire art continue to seek polymers and rubber compositions having an excellent balance of such properties.